1. Field of the Invention
This invention pertains to a seal for sealing an annular volume existing between an outer cylindrical member and an inner cylindrical member. More specifically, the invention pertains to an annular casing seal for use in an oil and gas well.
2. Description of the Prior Art
Ever since wells have been dug, or bored, into the earth for various purposes, the need for some means of supporting the wall of the well has been recognized. This need exists in the drilling of oil and gas wells, but in addition, some means is needed to protect the fluids within the wellbore and the penetrated formations from contamination or escape.
It is common practice in oil and gas well drilling to use strings of pipe, known as casing, suspended from the wellhead to isolate the well from the various formations. When it becomes desirable to have casing within the wellbore, a string of casing suspended from a casing hanger body is run into position in the well on a running tool as is well known in the art. Ordinarily, the casing will then be cemented in place to prevent migration of fluids along the outside of the casing. Cement is pumped down the wellbore and naturally flows around the bottom edge of the casing and back up the outside of the casing. The cement displaces the fluid outside of the casing forcing that fluid to flow around and over the casing hanger body and into the wellbore where it is removed to the drilling installation.
It is essential that an annular casing seal be used near the upper end of the casing in the wellhead to prevent fluids from either entering or leaving the wellbore around the top of the casing. So after the casing has been cemented in place, the annular casing seal is run into the wellhead and set. After sufficient drilling, it may become necessary to have another string of casing run into the well. The procedure described above would then be repeated.
Nearly all casing seals in use today utilize an elastomeric element to effect the seal between the outer surface of a casing hanger body and a larger cylindrical member, which could either be another casing hanger body or the wellhead housing. One such casing seal in use today is described in U.S. Patent 3,797,864.
Elastomeric seal elements have certain drawbacks. They tend to lose their effectiveness when exposed to heat or to corrosive substances such as hydrocarbons or hydrogen sulfide or to great pressures for long periods of time. Due to these causes, as well as natural aging, elastomers harden and become brittle. Once elastomeric seals lose their resiliancy they become very ineffective in preventing leaks.
Long exposures to high pressures not only tend to harden the elastomeric seal element, but the pressure can also cause the seal to flow and creep. As oil wells are drilled offshore in deeper water, the pressures on the casing seals increase dramatically. Since the wellhead is located at the bottom of the body of water, the casing seals will be subjected to great hydrostatic pressures. Besides the hydrostatic forces, which are of continuing nature, the formation fluids will occasionally generate extreme pressures within the wellbore which require the blowout preventers to be closed to control the well. The casing seals must contain the pressurized fluids within the wellbore. In these instances, the pressures applied to the casing seals can exceed 5,000 pounds per square inch.
It should also be noted that a certain amount of force is needed to set, or energize, a seal. In deep water offshore drilling, this force must be transmitted from the drilling unit at the surface of the body of water. Transmitting large, but controlled, forces from the surface to the wellhead to set a seal to resist great pressures can present formidable technical problems. Naturally, the greater the pressure an elastomeric nonpressure energized seal element must contain, the greater the pressure that will ordinarily be required to set the seal.
The casing seal disclosed in the aforementioned U.S. Patent 3,797,864, discloses a seal which is a combination elastomeric and metal to metal seal. The elastomeric element is the primary seal, but several metallic sealing lips are provided. The sealing lips are oriented in a manner such that fluid pressure acting on the seal will tend to force the sealing lips on the side of the seal affected by the pressure away from the sealing surfaces. However, if the elastomeric seal leaks and if the other or second set of metal sealing lips have previously been forced into contact with the annular walls, then this second set of lips will be pressure energized. In this sense, this could be a metal to metal pressure energized seal but whether the necessary forces to obtain the requisite initial metal to metal contact of the second set of sealing lips against the annular walls can be transmitted to those sealing lips is uncertain, especially if the elastomeric element has deteriorated to the point where it is leaking.
It can be seen that commonly used casing seals may not effectively and efficiently overcome the problems that exist in deep water offshore wells or in corrosive environments.